SPIRIFER2E OF THE UPPER HELDERBERG GROUP. 
199 
Shell large, ventricose, transverse, with the hinge-line usually less than 
the width of the shell; cardinal extremities rounded or truncate, hav¬ 
ing a subelliptical or subquadrate outline; mesial fold and sinus ex¬ 
treme. Surface plicated. 
Ventral valve variably convex on the two sides, with a wide mesial 
sinus, which is well defined in the upper part, becomes wider and 
deeper and less distinctly defined in the middle of the shell, and is 
produced in front into a long triangular extension; gently or more 
abruptly curving from the greatest convexity to the sides and cardinal 
angles : umbo prominent, with the apex incurved over the wide tri¬ 
angular fissure; area extending to the cardinal angles, with the mar¬ 
gin rounded except towards the extremities. 
Dorsal valve gibbous, highly elevated in the middle into a strong angu¬ 
lar mesial fold, and curving from the sides of the fold to the margins 
of the shell, except at the cardinal angles, where it is a little flattened 
and projecting, so as to give a minute auriculate appearance; summit 
of the mesial fold regularly arcuate from beak to base; apex slightly 
incurved over the narrow nearly vertical area. 
Surface, on either side of the mesial fold and sinus, marked by from 
sixteen to twenty plications, about four or five of which nearest the 
centre are dichotomous from below the middle of their length; ribs 
low, and rounded above, flattened below the middle, those towards the 
margin very slender : the first ten or twelve ribs on each side occupy 
the greater part of the valve. The entire surface is marked by delicate 
concentric striae, which are often crowded into imbricating lamellose 
lines towards the front of the shell. In very perfect specimens, these 
concentric striae are papillose Or fimbriated by fine radiating striae. 
These fine surface markings, however, are usually nearly or quite 
obliterated. 
The interior of a ventral valve shows but a partial thickening of the 
shell in the rostral cavity : the teeth are strong and short; the dental 
plates spreading and margining the upper part of the ovate muscular 
